[Track: qa-docs-ci] Task 216: Add build matrix: Linux + macOS

[benpayne]

Summary

Implement according to docs/implementation_plan.md task ID.

Source Task ID

• Plan ID: 216
• Section: Build and CI

Scope

• In scope:
  • Add build matrix: Linux + macOS
• Out of scope:
  • Unrelated parser/runtime refactors unless required

Dependencies

• Blocks:
• Blocked by:
• Related PRs/issues:

Acceptance Criteria

• Behavior implemented
• Tests added (pass/fail/e2e as appropriate)
• Existing tests pass
• Docs updated if user-facing

Test Plan

• Unit:
• Integration:
• Regression: